Method and circuit for adjusting a resistance in an integrated circuit

ABSTRACT

A method for adjusting a resistance in an integrated circuit, the resistance having a first conductive area and a second conductive area between which a dielectric area is arranged, a programming current being conducted through the resistance, the programming current being selected so as to adjust a resistance value of the resistance which is selected from a resistance range and is dependent on the programming current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims foreign priority benefits under 35 U.S.C. §119to co-pending German patent application 102 60 818.0, filed Dec. 23,2002. This related patent application is herein incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for adjusting a resistance in anintegrated circuit, particularly for adjusting the resistance value of afuse component. The invention also relates to a circuit by means ofwhich such a resistance can be adjusted.

2. Description of the Related Art

In integrated circuits, resistances are needed, the resistance values ofwhich must be adjusted more or less accurately. The resistances areimplemented in the integrated circuit, for example, by using channelswith corresponding doping. This leads to a large-area fixed resistancewhich is hard to control.

It is also possible to use transistors as resistance elements but notcompletely switching through the transistors. This leads to a nonlinearresistance characteristic and to a relatively large area being requiredfor this resistance.

In addition, high-resistance conductor tracks can be used which can betrimmed to a desired value by means of a laser. This is only possible aslong as the surface of the integrated circuit is accessible, that is tosay only in the case of unpackaged chips.

SUMMARY OF THE INVENTION

It is, therefore, the object of the present invention to provide aresistance component for integration in an integrated circuit, theresistance value of which can be adjusted in a simple and accuratemanner and which requires only a small area.

According to a first aspect of the present invention, a method foradjusting a resistance in an integrated circuit is provided. Theresistance exhibits a first conductive area and a second conductivearea, between which a resistance area is arranged. Such a structure canbe an antifuse structure or a migration fuse structure. A programmingcurrent is conducted through the resistance, the programming currentbeing selected so as to adjust a resistance value, dependent on theprogramming current, of the resistance, which resistance value isselected from a resistance range.

It has hitherto been normal practice to use an antifuse structure or amigration fuse structure for adjusting digital adjustment values in theintegrated circuit. For this purpose, the fuse structures are connectedto evaluating circuits which supply a digital value depending on thestate of the fuse structure.

An antifuse structure is initially not conductive. To program theantifuse structure, a programming current is conducted or not conductedthrough the antifuse structure. This results in two states of theantifuse structure: a first state in which the antifuse structure doesnot conduct and a second state in which the antifuse structureessentially has a low resistance. The two states correspond to theswitching states of the subsequent switch.

A migration fuse structure is initially at first conductive. To programthe migration fuse structure, a programming current is conducted or isnot conducted through the migration fuse structure. In a first state,when no programming current has been applied, the migration fusestructure is then conductive and in a second state, when a programmingcurrent has been applied, the migration fuse structure essentially has ahigh resistance. The two states correspond to the switching states ofthe subsequent switch.

The invention is based on the observation that different resistancevalues of the fuse structure can be adjusted depending on theprogramming current applied or, respectively, the duration of theprogramming current, i.e. the resistance values are freely selectable ina range of good conductance up to a range of insulation.

Due to the fact that the programming current determines the resistancevalue assumed by the fuse structure, a resistance value can bearbitrarily selected for the fuse structure from a range of resistances.It is possible to use both the antifuse structure and the migration fusestructure as a value-continuous resistance element, and not only as anelement for adjusting discrete adjustment values in an integratedcircuit.

In the range of adjustable resistances in the antifuse structure, thereis a range in which the adjustable resistance is essentially independentof an operating current. The operating current is the current which isimpressed into the resistance by a circuit in which the resistance isused. In this manner, a constant resistance can be ensured even when thecurrents of the resistance changes.

It is preferably provided that the programming current is applied to anexternal connection of the integrated circuit. This has the advantagethat internal generation of a programming current in the integratedcircuit is not required and thus a programming circuit necessary forthis can be saved in the integrated circuit.

It can also be provided that the programming current is generatedinternally in the integrated circuit so that the adjustment of theresistance values is performed without external help, for example duringthe first start-up.

According to a further aspect of the present invention, a circuit foradjusting a resistance in an integrated circuit is provided. Asdescribed above, the resistance is constructed as a fuse structure.Furthermore, a programming circuit is provided in order to conduct adefined programming current through the resistance in order to adjust aresistance value selected from a range of resistances.

The programming circuit is used for providing a programming currentwhich depends on the resistance to be adjusted.

The fuse structure is preferably constructed as an antifuse structurewhich has a dielectric area as resistance area. For this purpose, it canbe provided that the programming circuit has a current source in orderto conduct through the resistance a programming current which increaseswith time. When the predetermined resistance value is reached, thecurrent source can be switched off in order to retain the resistancevalue set. During the increase in programming current, the changingresistance value set in each case is continuously measured and it isdetermined, whether the set resistance value corresponds to the desiredresistance value to be set. If the desired resistance value has beenreached, the resistance value which has just been reached can beretained by switching off the current source. This makes it possible toimplement a programming circuit which is as simple as possible and bymeans of which a resistance formed from an antifuse structure can beadjusted.

Preferably, a measuring circuit is provided which detects when thepredetermined resistance has been reached and switches off the currentsource.

It can also be provided that the resistance range has a conductor areawhich is used as a migration area. The programming circuit selects thedefined programming current, and applies it for a particular period oftime, so that a defined decrease in a cross section of the conductorarea is achieved by electromigration. In this manner, the resistance ofthe resistance area can be increased by decreasing the cross section.The amount of resistance depends on the programming current applied andon the period of time for which the programming current is applied.

The programming circuit can preferably have a current source in order toconduct the programming current, at which electromigration occurs in theresistance area, through the resistance, wherein the current source canbe switched off when the predetermined resistance value is reached, inorder to retain the set resistance value. This defines the period oftime in which the programming current is applied.

According to a further aspect of the present invention, a method forusing an antifuse component as resistance element with a resistancewhich can be freely adjustable within a resistance range by means of aprogramming current is provided. This makes it possible to use antifusecomponents not only as permanent adjustment memories for adjustmentvalues but also for the resistance elements which are used in anintegrated circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

In the text which follows, preferred embodiments of the invention areexplained in greater detail by means of the attached drawings, in which:

FIG. 1 shows a possible structure of an antifuse component;

FIG. 2 shows the function of the resistance of the antifuse component independence on the current conducted through it and the parameter of theprogramming current;

FIGS. 3 a, 3 b show a migration fuse resistance;

FIG. 4 shows a programming circuit for an antifuse resistance accordingto an embodiment of the present invention; and

FIG. 5 shows a programming circuit for a migration fuse resistanceaccording to a further embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an antifuse structure. It has a first conductive layer 1which is preferably built from a metalizing layer. Above this, adielectric layer 2 is applied which has, e.g., a dielectric constant∈R=5.5. Naturally, dielectric layers having other dielectric constantscan also be used. The dielectric layer 2 can have silicon nitride inwhich oxygen molecules are implanted.

Above this, a second conductive layer 3 is deposited which is contactedvia a conductor connection 4. The second conductive layer 3 preferablyhas tungsten silicate but can also have other materials which areconductive. The conductor connection 4 is preferably constructed oftungsten, but other electrically conductive materials are also suitablefor it, such as, e.g., aluminum, copper etc.

After such an antifuse structure has been produced, the antifusestructure is initially nonconductive since a nonconductive dielectric isarranged between the first conductive layer 1 and the second conductivelayer 3. If a programming current is impressed into the antifusestructure, the breakdown voltage above the dielectric layer 2 isexceeded and a breakdown channel forms which changes the dielectriclayer 2 in such a manner that it retains a state in which it becomesconductive. The size of the breakdown channel is determined by theintensity of the programming current. The resistance which is adjustedabove the antifuse structure is dependent on the size of the breakdownchannel.

FIG. 2 shows a functional diagram which represents the relationshipbetween the resistance and the current conducted. The programmingcurrent is the parameter for the several functional curves. Function F1indicates the variation of resistance of the antifuse structure withprevious programming of the antifuse structure with a programmingcurrent of 0.2 mA. In a range between 0.2 mA and 0.3 mA, a continuouslydecreasing resistance can be seen, a programming current increasing inthis area causing a decrease in the resistance.

The second curve F2 shows the resistance curve of the antifuse structurewith currents between 0.0 mA to 0.4 mA with previous programming with0.3 mA and it can be seen that the variation of the resistance isgreatly dependent on the current. In the range between 0.3 mA and 0.4mA, a further drop in the resistance curve can be seen. In this area,the antifuse structure is programmed, i.e., the breakdown channel in theantifuse structure is widened or new breakdown channels are created sothat the electrical resistance is reduced.

The third function F3 then shows the resistance curve with a precedingprogramming current of 0.4 mA. In the range between 0.4 mA and 0.5 mA, alinear variation of the resistance with respect to the current I can beseen again in the third function F3. The monotonically decreasingvariation shows here, too, that the antifuse structure has beenprogrammed in this area, i.e., new breakdown channels have been formed.Thus, the greatest current in each case applied to the antifusestructure in each case determines the resistance curve in a range from 0mA up to the value of the greatest current hitherto applied.

In the diagram according to FIG. 2, nine functions are shown for ninedifferent programming currents and it can be seen that the greater theprogramming current was, the more constant is the variation of theresistance in the range between 0 mA and the maximum programming currentapplied in dependence on the operating current applied. In particular inthe range between 500 and 1000 Ω and with programming currents between1.5 mA and 2.5 mA, an almost constant variation of the resistance withrespect to the operating current applied can be seen. At the same time,a relatively flat variation of the decrease in resistance with respectto the programming voltage applied can be seen. This makes it possibleto adjust the resistance of the antifuse structure very accurately withthe aid of the programming current.

When the antifuse resistance is used in a later circuit, it is onlynecessary to pay attention to the fact that the normal operating currentof the integrated circuit does not exceed the programming current sinceotherwise the resistance of the antifuse structure would be changed. Inthe diagram according to FIG. 2, the operating current, or readoutcurrent, is identified by the vertical dashed line at 0.2 mA. It isparticularly in cases in which the operating current through theresistance fluctuates, e.g. in amplifier circuits, that it makes senseto design the antifuse structure in such a manner that relatively highprogramming currents can be used in order to achieve a linear variationof the resistance in the area of the operating current.

FIG. 3 a shows a migration fuse structure with a third conductive layer10 and a fourth conductive layer 11. Between the third and the fourthconductive layer 10, 11, a resistance area 12 is arranged which isconductive and which has a smaller cross section than the third andfourth conductive layer 10, 11. When a programming current is applied,the current density within the resistance area 12 is thus increasedcompared with the third and the fourth conductive area.

During the electromigration, a material transport of the conductormaterial takes place against the direction of the current at highcurrent densities which are material-dependent. In FIG. 3 b, the changein the migration fuse structure according to FIG. 3 a after aprogramming current has been applied for a particular period is shown.It can be seen that the cross section of the resistance area 12 isreduced in the vicinity of the lower potential and the cross section ofthe resistance area 12 is increased in the vicinity of the higherpotential. The reduction in cross section causes an increase inresistance, the cross section of the resistance area 12 which has beenachieved depending on the intensity of the electromigration effect. Theintensity of the electromigration can be determined essentially by themagnitude of the programming current and the duration for which theprogramming current has been applied over the resistance area. Theresistance of the migration fuse structure can thus be adjusted almostarbitrarily.

FIG. 4 shows a circuit for programming an antifuse resistance R1. Theantifuse resistance R1 is connected in series with a second resistanceR2. The series circuit of the two resistances is connected to acomparator circuit 5 in such a manner that the voltages dropped acrossthe antifuse resistance R1 and across the second resistance R2 arecompared with one another. A current I which is generated in a currentsource 6 is conducted through the series circuit of the resistances R1,R2.

It is the aim of the circuit to adjust the antifuse resistance R1 withthe aid of the resistance value of the second resistance R2. For thispurpose, the current source 6 outputs a slowly increasing current Iwhich causes programming of the antifuse resistance R1 and, at the sametime, voltage drops across the antifuse resistance R1 and the secondresistance R2. The voltage dropped across the antifuse resistance R1 isdetermined in accordance with the resistance value of the antifuseresistance R1 which depends on the programming current I.

If the same voltage is dropped across the two resistances R1, R2, thecomparator circuit 5 generates a stop signal which is applied to thecurrent source 6 via a signal line 7. In the current source 6, thecurrent is stopped from increasing as a consequence of the stop signal.This can be done by switching off the current source as a whole or bykeeping the impressed current at the level reached. As soon as the stopsignal causes the current I to stop increasing further, the resistancevalue of the antifuse resistance R1 is equal to the resistance value ofthe second resistance R2 at the current value I reached.

Depending on the magnitude of the programming current, the resistancevalue of the antifuse resistance R1 is essentially constant with thecurrent applied (see FIG. 2) so that this programmed antifuse resistanceR1 can be operated as fixed resistance in an integrated circuit.

To program the antifuse resistance R1, it can be provided that theremaining programming circuit with the second resistance of thecomparator circuit 5 and the current source 6 are provided outside theintegrated circuit and only the antifuse resistance R1 to be programmedcan be contacted via two connections. This makes it possible to savearea in the integrated circuit and, at the same time, to perform a veryaccurate adjustment of the antifuse resistance R1 since the secondresistance R2 can be predetermined very accurately, for example by usinga shunt resistance.

Naturally, it can also be provided that the comparator circuit 5 and thecurrent source 6 are arranged internally so that only a calibrationresistance in the form of the second resistance R2 has to be applied totwo external connections of the integrated circuit.

To adjust a migration fuse structure to a desired resistance value, asimilar circuit to the circuit according to FIG. 4 can essentially beused. Such a circuit is shown in FIG. 5. The migration fuse resistanceR3 is connected in series with a third resistance R4. The series circuitof the two resistances R3, R4 is connected to a comparator circuit 15 insuch a manner that the voltages dropped across the migration fuseresistance R3 and across the third resistance R4 are compared with oneanother. A current I which is generated in a second current source 16 isconducted through the series circuit, its migration fuse resistance R3and a third resistance R4.

Similar to the circuit according to FIG. 4, it is the aim of the circuitaccording to FIG. 5 to adjust the migration fuse resistance R3 to theresistance value of the third resistance R4. For this purpose, thesecond current source 16 outputs a current I which causes anelectromigration effect in the migration fuse resistance R3. At the sametime, the current I causes voltage drops across the migration fuseresistance R3 and the third resistance R4. The voltage dropped acrossthe migration fuse resistance R3 is determined in accordance with theresistance value of the migration fuse resistance R3 which depends onthe programming current I from a second current source 16.

If the same voltage is dropped across the two resistances R3, R4, thesecond comparator circuit 15 generates a stop signal which is appliedvia a second signal line 17 to the second current source 16, where theprogramming current is switched off as a consequence of the stop signal.In this manner, the resistance value just reached in the migration fuseresistance is not changed any further and is thus retained.

In contrast to the antifuse resistance, migration fuse resistances areessentially constant with the current applied so that there is no needto select the resistance range in order to achieve as constant aspossible a resistance variation with the operating current.

The advantage of using an antifuse structure as adjustable resistanceconsists in that, on the one hand, the resistance can be adjusted veryaccurately and, on the other hand, that an antifuse structure has a verysmall area requirement compared with high-resistance tracks which areusually integrated into an integrated circuit. In addition, it ispossible to program the antifuse resistance even after the integratedcircuit has been packaged.

1. A method for programming fuses, comprising: providing a fusecomprising a first conductive area, a second conductive area and aresistance area disposed between the first and second conductive areas;applying a programming current to the fuse for a period of time in orderto create a breakdown channel within the resistance area; andterminating the programming current upon achieving a desired resistancevalue of the fuse.
 2. The method of claim 1, further comprising, priorto applying the programming current, selecting the programming currenton the basis of an expected operating current to be applied to the fuseduring its use within an integrated circuit, wherein an expected peakvalue of the expected operating current is less than a peak value of theprogramming current.
 3. The method of claim 1, further comprising, priorto applying the programming current, selecting the desired resistancevalue for the fuse from a range of program-current-dependent resistancevalues.
 4. The method of claim 1, further comprising increasing theprogramming current over the period of time.
 5. The method of claim 1,wherein the fuse is an antifuse.
 6. The method of claim 1, wherein thefuse is a migration fuse.
 7. The method of claim 1, wherein the fuse isdisposed in an integrated circuit and the programming current is appliedto an external connection of the integrated circuit.
 8. The method ofclaim 1, wherein the fuse is disposed in an integrated circuit and theprogramming current is generated internally to the integrated circuit.9. A method for programming fuses, comprising: providing a fusecomprising a first conductive area, a second conductive area and aresistance area disposed between the first and second conductive areas;applying a programming current to the fuse for a period of time in orderto create a breakdown channel within the resistance area; measuring aresistance value of the fuse while applying the programming current,wherein the resistance value changes with continued application of theprogramming current; and terminating the programming current uponachieving a desired resistance value of the fuse.
 10. The method ofclaim 9, further comprising, prior to applying the programming current,selecting the programming current on the basis of an expected operatingcurrent to be applied to the fuse during its use within an integratedcircuit, wherein an expected peak value of the expected operatingcurrent is less than a peak value of the programming current.
 11. Themethod of claim 9, further comprising, prior to applying the programmingcurrent, selecting the desired resistance value for the fuse from arange of program-current-dependent resistance values.
 12. The method ofclaim 9, further comprising increasing the programming current over theperiod of time.
 13. The method of claim 9, wherein amperage of theprogramming current is held constant over the period of time andproduces an increasing resistance value.
 14. A fuse programmingapparatus, comprising: a fuse to be programmed and having a variableresistance value responsive to a programming current; a resistanceelement having a resistance value desired to be programmed into thefuse; a current source electrically connected to the fuse and theresistance element and configured to produce the programming current;and a comparator electrically connected to the fuse and the resistanceelement; wherein the comparator is configured to measure a voltage dropacross the fuse and the resistance element during application of theprogramming current from the current source, and is further configuredto cause the current source to terminate application of the programmingcurrent when a voltage drop across the fuse is substantially equal to avoltage drop across the resistance element.
 15. The apparatus of claim14, wherein the fuse is an antifuse.
 16. The apparatus of claim 14,wherein the fuse is a migration fuse.
 17. The apparatus of claim 14,wherein the current source is configured to increase the programmingcurrent during a period of time in which the fuse is being programmed.18. A circuit, comprising: a fuse comprising a first conductive area, asecond conductive area and a resistance area disposed between the firstand second conductive areas; and a programming circuit configured toconduct a controlled programming current through the resistance area ofthe fuse in order to adjust a resistance value of the fuse, theresistance value being selected from a range of resistance values. 19.The circuit of claim 18, wherein the resistance area comprises amigration area and wherein the programming circuit selects and appliesthe programming current for a particular period of time so as to achievea defined decrease in a cross-sectional area of the resistance area byelectromigration.
 20. The circuit of claim 18, wherein the programmingcircuit is configured to detect that the selected resistance value hasbeen achieved.
 21. The circuit of claim 18, wherein the programmingcircuit is configured to terminate the programming current upondetermining that the selected resistance value has been achieved. 22.The circuit of claim 18, wherein the programming circuit is configuredto increase the programming current during a period of time in which thefuse is being programmed with the selected resistance value.
 23. Thecircuit of claim 22, wherein the programming circuit is configured toterminate the programming current upon determining that the selectedresistance value has been achieved.
 24. A method for adjusting aresistance in an integrated circuit, comprising: providing a fusecomprising a first conductive area, a second conductive area and aresistance area between the first and second conductive areas; selectinga resistance value of the resistance area from a range of resistancevalues, each value being dependent on a respective programming current;selecting a programming current on the basis of the selected resistancevalue; and applying the programming current to the resistance area tocause a change in a resistance of the resistance area until the selectedresistance value is achieved.